Winter Storm Warning 8, NWS Reno, NV

Area Forecast Discussion
National Weather Service Reno NV
310 PM PST Wed Mar 8 2023

.SYNOPSIS...

After a short break in storm activity through Thursday morning, a
potent atmospheric river storm will bring rising snow levels with
strong winds, heavy mountain snow and lower elevation rain
Thursday afternoon through Friday, with possible flooding in poor
drainage areas. Additional waves of moisture will keep rain and
snow chances going at times from this weekend into next week.

&&

.DISCUSSION...

Some bands of light snow showers will linger into the evening
across parts of western Nevada along the I-80 corridor to the
north and south to Highway 50. Snow accumulations will remain
generally light with only minor impacts for roads already cleared
of snow. Breezy conditions accompanying the short wave trough
passage, along with areas of blowing dust across the Basin and
Range north of I-80 and east of Highway 95 will begin to diminish
into the evening. A quick moving short wave ridge will then bring
a brief period of clearing overnight into Thursday morning before
the next winter system moves into the region later in the
afternoon. -Amanda

A very active and impactful weather pattern for the upcoming week
will be highlighted by a surge of atmospheric river moisture
accompanied by a 150 kt subtropical jet headed directly into the
Sierra starting Thursday afternoon, then spreading across western
NV. Peak precip rates, wind speeds and impacts are expected
Thursday evening through Friday afternoon. Additional waves of
moisture, although not as intense or warm, are projected to
continue through the weekend.

Highlights for the first storm from Thursday night through the
weekend:

* WINDS: 700 mb flow of 60-70 kt, with jet stream level flow of
  120-130 kt over the Sierra, will bring strong and potentially
  damaging winds late Thursday evening through Friday evening. The
  peak gusts are likely to reach western NV during the day Friday.
  Even with some dampening of the wind speeds with precip
  occurring, wind gusts of 60-70 mph remain possible at times,
  with Sierra ridge gusts surpassing 120 mph.

* PRECIP AMOUNTS: Impressive precipitation totals are projected
  from Thursday afternoon through Saturday night, with recent
  guidance indicating 6-9" of liquid equivalent along the Sierra
  crest, 3-6" in Sierra communities, 1-2" along US-395 from
  northeast CA to the main urban areas of far western Nevada, 2-4"
  for foothill areas mainly west of US-395, and 0.25-0.5" for the
  west central NV Basin & Range. The latter portion of the
  weekend could bring another 0.50" liquid to the Tahoe area with
  around 1" near the Sierra crest, but western NV likely becomes
  more shadowed out with minimal additional rainfall through
  Sunday. These precip amounts will be capable of producing
  flooding concerns--see the hydrology segment for additional
  details.

* SNOW LEVELS/AMOUNTS: This continues to be the wild card as snow
  levels start rather low (near 4500-5500 feet) with the initial
  moisture push Thursday afternoon. The most likely scenario
  raises snow levels quickly through Thursday evening-night to
  near 8000-8500 feet. While more intense precip could
  occasionally lower snow levels by 1000-1500 feet, the majority
  of precip below 7500-8000 feet will most likely fall as rain or
  a rain-snow mix until some cooling arrives Friday afternoon-
  night. Snow levels then look to dip to 5000-6000 feet by early
  Saturday for Tahoe and western NV between I-80 and US-50, near
  7000 feet for Mono County and western NV south of US-50, but
  fall down to near 4000 feet for northeast CA-far northwest NV
  where the colder air mass makes more progress. The cold front
  then stalls and lift northward before becoming more poorly
  defined during the remainder of the weekend, with resultant snow
  levels generally around 5500-6000 feet north of I-80 and
  6000-7000 feet from I-80 southward. Snow amounts for the Sierra
  will vary quite widely by elevation but with expectations for
  several feet in the higher elevation sites, the watches for the
  eastern Sierra/Tahoe area will be upgraded to Winter Storm
  Warnings--see our latest statement for more details on potential
  snowfall amounts. For the main western NV urban areas, this
  event will be nearly all rain, with a few foothill sites above
  5000-5500 feet maybe receiving up to an inch or so of snow at
  the onset (Thursday evening) and the tail end (early AM
  Saturday) of the main storm.

For early-mid next week, the Sierra may not see much of a break as
upslope snow showers continue Sunday night-Monday. Then another
colder storm system moves through with a round of moderate-heavy
snow Monday night into Tuesday. Even beyond that, another fast
moving storm is on track for midweek. While these storms currently
appear to be colder compared to the Thursday-Saturday event and
reduce the overall flood risk, the tracks still could vary and
bring a period of higher snow levels. However, the colder storm
scenario would further compound the winter impacts for areas
already overwhelmed by excessive season-long snowfall, including
the new snow coming prior to next week. For lower elevations of
western NV, significant snowfall isn't likely, but short bursts
of snow could produce some travel impacts for the Tuesday and
Wednesday morning commutes. MJD

&&

.AVIATION...

* Some light snow showers will linger in the vicinity of Sierra
  terminals with bands of light snow showers diminishing across
  western Nevada going into the evening. A lingering light snow
  shower or two may briefly bring MVFR/IFR conditions in CIG/VIS
  for KLOL and KNFL. For the most part VFR conditions will prevail
  for western Nevada terminals overnight through Thursday morning.
  Later Thursday morning, snow shower chances will begin to
  increase across northeast California, which will spread periods
  of IFR conditions east and south towards Sierra terminals (KSVE-
  KTRK-KTVL) Thursday afternoon.

* Moderate turbulence and LLWS will subside briefly overnight but
  begin to increase once more late Thursday morning as Sierra
  ridgetop wind gusts surge to 90 kts. FL100 winds will also
  increase to 50-60 kts by 00Z Friday as well. Sierra terminals
  will see southwest wind gusts increasing to 20-30 kt Thursday
  afternoon and evening, with western Nevada terminals seeing
  wind gusts to 25 kts.

* Latest model scenarios continue to show a very active pattern
  starting Thursday night though Saturday and a followup lower
  confidence storm Monday through Wednesday next week. Aviators
  should expect periods of strong winds and turbulence, along with
  heavy precipitation to impact the region Thursday night-Saturday
  and again Monday to Wednesday next week. -Amanda

&&

.AVALANCHE WEATHER...

High confidence for a strong atmospheric river storm Thursday
night through Saturday, producing very heavy precipitation with
variable snow levels and significant impacts for avalanche
concerns.

* 24-hr Snow Water Equivalent (SWE): High-end SWE of 3-6" along
  the crest from Thursday evening through Friday afternoon, with
  hourly rates of 0.25-0.50" at times during peak intensity. SWE
  rates decrease by Friday night but additional 1-3" possible
  through the weekend.

* Snow-Liquid Ratios: Lower snow-liquid ratios of 7-10 to 1,
  except 10-13 to 1 for the Sierra crest above 9000 ft through
  Friday. Cooler air will increase snow-liquid ratios to 10-14 to
  1 for Friday night-Saturday morning.

* Winds: Strong southwest winds expected with gusts to 100-125
  mph across the ridges Thursday afternoon through Saturday
  morning.

Active pattern will continue through the middle of next week with
the highest probability for more impactful snow and wind Monday
night-Wednesday. MJD

&&

.HYDROLOGY...

Bottom Line: Increasing confidence in flooding along creeks and
streams and areas of poor drainage where deep snow may confine
flow and limit drainage. Significant rises are expected on main
stem rivers. While flooding is not currently forecast, rises to
monitor stage are now forecast for several points, and minor to
moderate flooding can't be ruled out if the system overperforms
in terms of precipitation amounts and/or prolonged periods of
higher snow levels occur.

The flood threat does not go away Sunday. Any additional rains next
week could lead to additional rises, and some rivers will be slow
to respond to the Thursday-Saturday event.

Late Thursday through Saturday, rain combined with strong winds
and high humidity will add considerable energy into our record to
near-record deep regional snowpack at mid to lower elevations.
While higher elevation and deepest snowpacks will continue to
accumulate snow and water equivalent, deep mid-elevation
snowpacks are expected to retain most of the projected rain. Some
of these areas may begin to develop preferential pathways for melt
and start releasing rainfall after prolonged periods of rain, but
are unlikely to contribute significant snowmelt to the runoff. It
is the areas of relatively shallow snowpack on the order of
inches to a couple of feet that we are most concerned about having
significant melt and contributing to the flood concerns.
Fortunately, these areas are confined to low elevation drainages
and to limited portions of the main stem rivers.

Our greatest concern continues to be creeks and streams draining
large areas of terrain with shallow snowpacks (below ~5500 feet)
and urban poor drainage areas where deep snow may be restricting
drainage. Confidence is increasing in flood impacts for these
areas.

The main-stem rivers forecasts have ramped up somewhat, but still
remain below flood stages. Keep a close eye on the Pit River into
Big Valley, the Susan River, The Middle Fork of the Feather, and
the forks and main stem of the Carson River. The Susan River will
respond very rapidly to heavy rain, while the response for the
Pit River into Big Valley and the Middle Fork of the Feather may
take several days. The latest guidance from the CNRFC now has the
East fork of the Carson and the main stem Carson River near
Carson City exceeding monitor stages.

The potential for additional rainfall early next week will require
close monitoring as the Thursday - Saturday system will help prime
the snowpack and stream responses. Check back for forecast
updates at cnrfc.noaa.gov. Changes in storm track could lead to
changes in precipitation and or rain snow elevation that could
exacerbate or mitigate flooding concerns. -Bardsley

&&

.REV Watches/Warnings/Advisories...
NV...High Wind Watch from Thursday evening through Friday evening
     NVZ001-003>005.

     Flood Watch from Thursday afternoon through Sunday morning
     NVZ001>003.

     Winter Storm Warning from 4 PM PST Thursday to 11 AM PDT Sunday
     NVZ002.

     Lake Wind Advisory until 6 PM PST this evening NVZ004.

CA...High Wind Watch from Thursday evening through Friday evening
     CAZ070.

     Flood Watch from Thursday afternoon through Sunday morning
     CAZ071>073.

     Winter Storm Warning from 4 PM PST Thursday to 11 AM PDT Sunday
     CAZ071.

     Winter Storm Warning from 4 PM PST Thursday to 11 AM PDT Sunday
     CAZ072.

     Winter Storm Warning from 4 PM PST Thursday to 11 AM PDT Sunday
     CAZ073.

&&

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